My local Petco has plants that I buy. They have several different kinds and they are about $2 each.
You need a plant that is pretty tough as snakes can be pretty rough on them sometimes. My GTP is super rough on his while my Ball is very gentle. I would not buy anything to expensive or elaborate until you know how your snake will treat them.
KMG
0.1 BP 1.1 Blood Python 1.0 Brazilian Rainbow Boa 1.0 Aru Green Tree Python
0.1 Emerald Tree Boa 0.1 Dumeril Boa 0.1 Carpet Python 0.1 Central American Boa
0.1 Brooks Kingsnake 0.1 Speckled Kingsnake 1.0 Western Hognose
0.1 Blonde Madagascar Hognose 1.0 Columbian Boa
Pothos is a good choice. What it lacks in sturdiness it more than makes up for with growth rates and ease of propagating. Simply cut off several inches of the vine and plunk it in a glass of water. You'll have roots shortly, stick rooted cuttings in dirt and you're off to the races. With a bit of practice you should be able to replace plants as your snake destroys them from your own cuttings.